Optical modulator

ABSTRACT

An optical modulator suppresses a radiation mode of a microwave generated in a connection substrate or termination substrate from entering a signal electrode of the optical modulator and suppresses modulation properties from being degraded. The optical modulator includes an optical modulation element ( 1 ) having a substrate with an electro-optic effect, an optical waveguide formed on the substrate, and a modulating electrode (or a signal electrode ( 2 )) for modulating light passing through the optical waveguide; and a connection substrate ( 4 ), arranged outside the substrate, for supplying the optical modulation element with a microwave signal operating the optical modulation element. A signal input terminal ( 22 ) and a signal output terminal ( 23 ) are formed on the connection substrate ( 20 ). A recombination suppression unit suppresses a radiation mode ( 24 ) of the microwave signal input to the signal input terminal ( 22 ) from being recombined with the signal output terminal ( 23 ) provided on the connection substrate.

This application is a division of U.S. patent application Ser. No.12/085,115, filed May 16, 2008 (with a §371 (c) date of Jan. 9, 2009),which was the U.S. national phase of PCT/JP2006/322533 filed Nov. 13,2006, and claims priority of Japan Pat. App. No. 2005-331954 filed Nov.16, 2005, the contents of all of which are incorporated herein byreference. The present invention relates to an optical modulator, andmore particularly, to an optical modulator having a connection substratearranged outside an optical modulation element or a terminationsubstrate.

TECHNICAL FIELD Background Art

In general, waveguide type optical modulators in which an opticalwaveguide or a modulating electrode is formed onto a substrate having anelectro-optic effect are widely used in optical communication fields oroptical measurement fields. At the request of improvement of high-speed,high-capacity communication or optical measurement precision in thisoptical modulator, the development of an optical modulator stablyoperable even in a high-frequency band is required. Recently, opticalmodulators of more than several tens GHz have also been implemented.

When the optical modulator operates in the high-frequency band, a jittervalue of an eye pattern is apt to be large in an optical output waveformfrom the optical modulator. There occurs the degradation of modulationproperties including the degradation of waveform quality of an opticaloutput signal or the reduction of an optical transmission distance.

In a result of keen research by the inventors, it has been found thatnoise included in a microwave signal operating the optical modulator isone of factors causing the jitter value to be large as described below.

An example of the optical modulator is shown in FIG. 1. An opticalmodulation element 1 of FIG. 1( a) is formed with an optical waveguide(not shown), a modulating electrode, and the like on a substrate havingthe electro-optic effect such as LiNbO₃. The modulating electrode isconstructed with a signal electrode 2, a ground electrode (not shown),and the like. The optical modulation element 1 is connected to anoptical fiber 3 for receiving and emitting an optical wave.

A connection substrate 4 having an amplifier 8 and the like and atermination substrate 5 having a termination device 9 and the like arearranged around the optical modulation element 1. Along with the opticalmodulation element 1, the connection substrate 4 or the terminationsubstrate 5 is accommodated within a case 10 and forms an opticalmodulator module.

For reference, an example of the optical modulator module using theconnection substrate is disclosed in Patent Document 1.

[Patent Document 1] JP-A-2003-233043

A method of operating an optical modulator will be described. Amicrowave signal generated from a modulation signal source 6 isintroduced into a GPO connector 7 corresponding to an input terminal ofa case 10 and is transmitted form the associated connector to a signalinput terminal 11 of a connection substrate 4 as shown in FIG. 1B.

In the connection substrate 4, the microwave signal is output to asignal output terminal 12 through an amplifier 8 or a functional element(not shown) for converting the microwave signal into various states.

Wire bonding is done between the signal output terminal 12 of theconnection substrate and an electrode pad of the signal electrode 2 ofan optical modulation element. The microwave signal output from theconnection substrate 4 is continuously transmitted to the signalelectrode 2. According to the microwave signal transmitted to the signalelectrode 2, the optical wave propagating within the optical waveguideof the optical modulation element is optically modulated.

An additional electrode pad is provided on a terminal of the signalelectrode 2. Similarly, wire bonding is done between the electrode padand a signal introduction terminal 14 of the termination substrate asshown in FIG. 1C. Thus, the microwave signal is additionally transmittedfrom the signal electrode 2 to the termination substrate 5, and isabsorbed by a termination device 9 provided within the terminationsubstrate.

However, the inventors have found that a radiation mode 13 of amicrowave is generated from a microwave signal input to the signal inputterminal 11 in the connection substrate 4 as shown in FIG. 1B and theradiation mode 13 propagates through the connection substrate and isrecombined with the microwave signal propagating through a signal linein the signal output terminal 12. The recombined radiation mode servesas noise in a modulation signal. This noise propagates through thesignal electrode 2 of the optical modulation element, thereby degradingmodulation properties of the optical modulator.

In the termination substrate 5 as shown in FIG. 1C, part of a microwavesignal introduced into the termination device 9 is reflected by thetermination device and generates a radiation mode 15 of the microwave.The radiation mode 15 is recombined with the signal introductionterminal 14 of the signal propagating through the termination substrate,and is propagated to the signal electrode 2, so that the microwavetravels in a direction reverse to the conventional propagationdirection. This radiation mode 15 also serves as noise in a modulationsignal.

In the connection substrate 4, a radiation mode (not shown) of themicrowave reflected by the signal output terminal 12 is generated. Themicrowave propagating through the signal electrode 2 in the reversedirection generates a radiation mode (not shown) in the signal outputterminal 12. These radiation modes are recombined with the signal inputterminal 11 of the signal propagating through the connection substrateand flow backward to the modulation signal source 6, thereby causing theoperation of the optical signal source to be unstable.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The invention has been made to solve the above-mentioned problems, andit is an object of the invention to provide an optical modulator thatsuppresses a radiation mode of a microwave generated in a connectionsubstrate or a termination substrate from entering a signal electrode ofthe optical modulator and suppresses modulation properties from beingdegraded.

Means for Solving the Problems

According to a first aspect of the invention, an optical modulatorincludes an optical modulation element having a substrate with anelectro-optic effect, an optical waveguide formed on the substrate, anda modulating electrode for modulating light passing through the opticalwaveguide; and a connection substrate, arranged outside the substrate,for supplying the optical modulation element with a microwave signaloperating the optical modulation element, wherein a signal inputterminal and a signal output terminal are formed on the connectionsubstrate, and a recombination suppression unit that suppresses aradiation mode of the microwave signal input to the signal inputterminal from being recombined with the signal output terminal isprovided on the connection substrate.

According to a second aspect of the invention, in the optical modulatoraccording to the first aspect, the recombination suppression unit isarranged so that the signal output terminal is not located on a paththrough which the radiation mode of the microwave signal propagates.

According to a third aspect of the invention, in the optical modulatoraccording to the first aspect, the recombination suppression unit isformed around the signal output terminal and is a unit that shields theradiation mode of the microwave signal or discharges the radiation modeof the microwave signal outside the connection substrate.

According to a fourth aspect of the invention, in the optical modulatoraccording to any one of the first to third aspects, a functional elementfor converting the microwave signal into various states is arranged onthe connection substrate.

In the invention, “the functional element for converting the microwavesignal into various states” is arranged on the connection substrate, andrefers to an electrical circuit element having a function for convertinga microwave signal state into a specific state in signalamplification/attenuation, phase adjustment, signal division orcombination, or the like according to the microwave signal.

According to a fifth aspect of the invention, an optical modulatorincludes an optical modulation element having a substrate with anelectro-optic effect, an optical waveguide formed on the substrate, anda modulating electrode for modulating light passing through the opticalwaveguide; and a termination substrate, arranged outside the substrate,for supplying a microwave signal operating the optical modulationelement from the optical modulation element to a termination device,wherein a signal introduction terminal and a termination device areformed to the termination substrate, and a recombination suppressionunit that suppresses a radiation mode of the microwave signal reflectedfrom the termination device from being recombined with the signalintroduction terminal is provided on the termination substrate.

According to a sixth aspect of the invention, in the optical modulatoraccording to the fifth aspect, the recombination suppression unit isarranged so that the signal introduction terminal is not located on apath through which the radiation mode of the reflected microwave signalpropagates.

According to a seventh aspect of the invention, in the optical modulatoraccording to the fifth aspect, the recombination suppression unit isformed around a signal output terminal and is a unit that shields theradiation mode of the reflected microwave signal or discharges theradiation mode of the reflected microwave signal outside the terminationsubstrate.

EFFECTS OF THE INVENTION

According to a first aspect of the invention, since an optical modulatorincludes an optical modulation element having a substrate with anelectro-optic effect, an optical waveguide formed on the substrate, anda modulating electrode for modulating light passing through the opticalwaveguide; and a connection substrate, arranged outside the substrate,for supplying the optical modulation element with a microwave signaloperating the optical modulation element, wherein a signal inputterminal and a signal output terminal are formed on the connectionsubstrate, and a recombination suppression unit that suppresses aradiation mode of the microwave signal input to the signal inputterminal from being recombined with the signal output terminal isprovided on the connection substrate, a defect in which the radiationmode is recombined with the signal output terminal and enters a signalelectrode can be suppressed and the degradation of modulation propertiesdue to the radiation mode of the microwave can be suppressed.

According to a second aspect of the invention, since the recombinationsuppression unit is arranged so that the signal output terminal is notlocated on a path through which the radiation mode of the microwavesignal propagates, a defect due to the radiation mode can be effectivelyaddressed in a simple and convenient configuration in which anarrangement of the signal input terminal generating the radiation modeand the signal output terminal recombined with the radiation mode isadjusted.

According to a third aspect of the invention, since the recombinationsuppression unit is formed around the signal output terminal and is aunit that shields the radiation mode of the microwave signal ordischarges the radiation mode of the microwave signal outside theconnection substrate, a recombination of the radiation mode with thesignal output terminal can be effectively suppressed by forming theshielding or discharging unit to the connection substrate.

According to a fourth aspect of the invention, since a functionalelement for converting the microwave signal into various states isarranged on the connection substrate, a defect due to the radiation modecan be addressed also in various connection substrates.

According to a fifth aspect of the invention, since an optical modulatorincludes an optical modulation element having a substrate with anelectro-optic effect, an optical waveguide formed on the substrate, anda modulating electrode for modulating light passing through the opticalwaveguide; and a termination substrate, arranged outside the substrate,for supplying a microwave signal operating the optical modulationelement from the optical modulation element to a termination device,wherein a signal introduction terminal and a termination device areformed to the termination substrate, and a recombination suppressionunit that suppresses a radiation mode of the microwave signal reflectedfrom the termination device from being recombined with the signalintroduction terminal is provided on the termination substrate, a defectin which the radiation mode of the microwave signal reflected from thetermination device is recombined with the signal introduction terminaland enters a signal electrode can be suppressed and the degradation ofmodulation properties due to the radiation mode of the microwave can besuppressed.

According to a sixth aspect of the invention, since the recombinationsuppression unit is arranged so that the signal introduction terminal isnot located on a path through which the radiation mode of the reflectedmicrowave signal propagates, a defect due to the radiation mode can beeffectively addressed in a simple and convenient configuration in whichan arrangement of the termination device and the signal introductionterminal is adjusted.

According to a seventh aspect of the invention, since the recombinationsuppression unit is formed around a signal output terminal and is a unitthat shields the radiation mode of the reflected microwave signal ordischarges the radiation mode of the reflected microwave signal outsidethe termination substrate, a recombination of the radiation mode withthe signal introduction terminal can be effectively suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional optical modulator.

FIG. 2 is a schematic diagram of an optical modulator in accordance withthe invention.

FIG. 3 is a diagram showing examples in which a plurality of signalinput terminals or a plurality of signal output terminals are providedin the optical modulator in accordance with the invention.

FIG. 4 is a diagram showing examples of using a shielding body in theoptical modulator in accordance with the invention.

FIG. 5 is a diagram showing examples of using a radiation modedischarging unit in the optical modulator in accordance with theinvention.

FIG. 6 is a diagram showing examples of using a through hole as theradiation mode discharging unit in the optical modulator in accordancewith the invention.

REFERENCE NUMERALS

-   1: OPTICAL MODULATION ELEMENT-   2: SIGNAL ELECTRODE-   3: OPTICAL FIBER-   4, 20, 30, 40, 50, 60, 70: CONNECTION SUBSTRATE-   5, 21: TERMINATION SUBSTRATE-   6: MODULATION SIGNAL SOURCE-   7: CONNECTOR-   8, 52, 63, 72: AMPLIFIER-   9: TERMINATION DEVICE-   10: CASE-   11, 22, 31, 41, 42, 51, 61, 71: SIGNAL INPUT TERMINAL-   12, 23, 33, 34, 44, 45, 54, 65, 74: SIGNAL OUTPUT TERMINAL-   13, 15, 24, 26, 35, 46, 47, 56, 67, 76: MICROWAVE RADIATION MODE-   14, 25: SIGNAL INTRODUCTION TERMINAL-   53, 62, 64, 73: SIGNAL LINE-   55: SHIELDING BODY-   66: CAVITY (CONCAVE PORTION)-   75: THROUGH HOLE

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, exemplary embodiments of the invention will be described.

FIG. 2 is a schematic diagram of an optical modulator in accordance withthe invention.

In FIG. 2, portions having the same reference numerals as those of FIG.1 have configurations similar thereto. In the invention, if a substratehaving an electro-optic effect, an optical waveguide formed onto thesubstrate, and a modulating electrode for modulating light passingthrough the optical waveguide are provided for an optical modulationelement 1, they are not specifically limited in terms of materials orother structures. For example, the substrate having the electro-opticeffect can use lithium niobate, lithium tantalate, Lead LanthanumZirconate Titanate (PLZT), and a quartz-based material. The opticalwaveguide of the substrate can be formed by diffusing Ti or the like onthe substrate surface in a thermal diffusion method, a proton exchangemethod, or the like. A signal electrode or a ground electrodeconstructing the modulating electrode can be formed by an electrodepattern of Ti/Au and can be formed by a gold plating method or the like.If needed, a buffer layer of a dielectric substance of SiO₂ or the likecan be provided on the substrate surface after the optical waveguide isformed.

A feature of the invention shown in FIG. 2 is that a microwave radiationmode can be effectively suppressed from being recombined with a signalline by adjusting an arrangement of the input and output terminals andthe like within the substrate as shown in FIG. 2( b) or 2(c) in aconnection substrate 20 and a termination substrate 21.

In the connection substrate 20, a radiation mode 24 of the microwavesignal generated from a signal input terminal 22 is radiated at an angleof more than about one degree with respect to the signal line and almostpropagates through the connection substrate in a direction as indicatedby a dashed dotted line A. Thus, the radiation mode 24 can be suppressedfrom being recombined with a signal output portion 23 by arranging asignal output portion 23 in a position away from the dashed dotted lineA.

Similarly, in the termination substrate 21, a radiation mode 26 of themicrowave-reflected from a termination device 9 almost propagatesthrough the termination substrate in a direction as indicated by adashed dotted line B. Thus, the radiation mode 26 can be suppressed frombeing recombined with a signal introduction terminal 25 by arranging thesignal introduction terminal 25 away from the dashed dotted line B.

By adopting the configuration as shown in FIG. 2( b) also for aradiation mode (not shown) of the microwave generated from the signaloutput terminal 23 as described above in the connection substrate 20,the radiation mode can be suppressed from being recombined with thesignal input terminal 22 and the influence to a modulation signal sourcecan be mitigated.

In FIG. 2, an example is shown in which one signal input terminal 22 andone signal output terminal 23 are located in the connection substrate20, but the optical modulator according to the invention is not limitedthereto. For example, a plurality of signal output terminals can beincluded in a connection substrate 30 as shown in FIG. 3( a).Alternatively, a plurality of signal input terminals and a plurality ofsignal output terminals can be included in a connection substrate 40 asshown in FIG. 3( b).

In the case of FIG. 3( a), signal output terminals 33 and 34 arearranged in positions away from a propagation direction (as indicated bya dashed dotted line C) for a radiation mode 35 of a microwave signalradiated from a signal input terminal 31.

By arranging signal output terminals 44 and 45 in positions away fromdashed dotted lines D and E for a radiation mode 46 of a microwavesignal radiated from a signal input terminal 41 and a radiation mode 47of a microwave signal radiated from another signal input terminal 42 inthe case of FIG. 3( b) the radiation modes can be suppressed from beingrecombined with the signal output terminals. Reference numeral 43 refersto a compound functional element in which a combiner or divider and thelike are combined.

The connection substrate has been described with reference to FIG. 3.Similarly, it is also possible to construct a termination substrate inwhich multiple signal introduction terminals or multiple terminationdevices are used.

In FIGS. 2 and 3, a method of adjusting an arrangement of the input andoutput terminals and the like has been illustrated as a recombinationsuppression unit for suppressing a recombination of the microwaveradiation mode, but the invention is not limited thereto. As shown inFIG. 4 or 5, there can be used a unit for shielding the microwaveradiation mode or discharging the microwave radiation mode outside thetermination substrate.

FIG. 4 shows examples of a connection substrate 50 having an amplifier52. A configuration is made so that a radiation mode 56 of a microwaveradiated from a signal input terminal 51 is shielded using a shieldingbody 55 and is not incident into a signal output portion 54. FIG. 4( b)shows a sectional view taken along a dashed dotted line F of FIG. 4( a).Reference numeral 53 refers to a signal line.

Preferably, a low dielectric loss material such as aluminum, oxidealuminum, or the like can be used as a substrate material constructing aconnection substrate or a termination substrate. Preferably, aconductive metal material such as Au, Al, or the like can be used as theshielding body.

A method of assembling the shielding body in a substrate can adopt amethod of forming a concave portion to the substrate in a cuttingprocess or the like, a method of inserting a small piece made of ashielding material into the concave portion, or the like.

FIG. 5 shows examples of a connection substrate 60 having an amplifier63. A configuration is made so that a radiation mode 67 of a microwaveradiated from a signal input terminal 61 is discharged outside theconnection substrate 60 using a cavity (or concave portion) 66 and isnot incident into a signal output portion 65. FIG. 5( b) shows asectional view taken along a dashed dotted line G of FIG. 5( a).Reference numerals 62 and 64 refer to signal lines.

As shown in FIG. 6, a radiation mode 76 of a microwave radiated from asignal input terminal 71 can be discharged outside a connectionsubstrate 70 using a through hole 75 passing through the connectionsubstrate 70 in the connection substrate 70 having an amplifier 72. Thisconfiguration is made so that the radiation mode 76 of the microwave isnot incident into a signal output portion 74. FIG. 6( b) shows asectional view taken along a dashed dotted line H of FIG. 6( a).Reference numeral 73 refers to a signal line.

The technique of shielding or discharging the radiation mode as shown inFIGS. 4 to 6 can be applied to a connection substrate or a terminationsubstrate having other functional elements. If needed, various types oftechniques as described above can be also combined and used.

INDUSTRIAL APPLICABILITY

According to the invention as described above, an optical modulator canbe provided which suppresses a radiation mode of a microwave generatedin a connection substrate or a termination substrate from entering asignal electrode of the optical modulator and suppresses modulationproperties from being degraded.

1. An optical modulator comprising: an optical modulation element havinga substrate with an electro-optic effect, an optical waveguide formed onthe substrate, and a modulating electrode for modulating light passingthrough the optical waveguide; and a termination substrate, arrangedoutside the substrate, for supplying a microwave signal operating theoptical modulation element from the optical modulation element to atermination device, wherein a signal introduction terminal and atermination device are formed to the termination substrate, and arecombination suppression unit is provided on the termination substrate,and said recombination suppression unit suppresses a radiation mode of amicrowave signal reflected from the termination device from beingrecombined with the signal introduction terminal.
 2. The opticalmodulator according to claim 1, wherein the recombination suppressionunit is arranged so that the signal introduction terminal is not locatedon a path through which the radiation mode of the reflected microwavesignal propagates.
 3. The optical modulator according to claim 1,wherein the recombination suppression unit is formed around a signaloutput terminal and is a unit that shields the radiation mode of thereflected microwave signal.
 4. The optical modulator according to claim1, wherein the recombination suppression unit is formed around a signaloutput terminal and is a unit that discharges the radiation mode of thereflected microwave signal outside the termination substrate.